Onions like well-drained soil with a pH between 6.0 and 7.0, and they like it full of tasty organic matter. Whether and how much you fertilize your onions depends on the composition of your soil. The first step to understanding the nutrient content of your soil is to test it.
If you can’t get your soil tested, then just use a balanced fertilizer, like 8-8-8 or 10-10-10. Onions need a good supply of nitrogen—but not too much. Too much nitrogen can make the plant take longer to mature. It can also cause soft bulbs, which will greatly decrease the onion’s storage life.
Side dress your onions after the root systems are well developed. Spread the fertilizer along your onion row, and gently work it in with a shallow rake or gardening fork. Whatever you do, don’t use any of those “weed and feed” fertilizers. They contain weed killers that can kill your onions and other vegetables in your garden.
Do you fertilize your onion plants? What type of fertilizer do you use? How often do you fertilize? Please tell us exactly how and when you fertilize your onion plants.